The present invention relates to an electrophotographic photoreceptor of a function separation type which comprises a substrate, a carrier generation layer, a carrier transport layer, a surface protective layer, and a carrier injection regulating layer between the charge generation and surface protective layer. The photoreceptor of the invention is further provided with a thermal expansion relieving layer between the carrier injection regulating layer and the surface protective layer.
In a printer of an electrophotographic system, light of long wavelength, such as 630 to 800 nm, projected from an exposure source such as a light emitting diode, a semiconductor laser or a gas laser, is used as writing light for forming an electrostatic latent image on the surface of a photoreceptor. In such a printer, a function separation type photoreceptor composed of a carrier generation layer which has a high sensitivity even to long wavelength light, a carrier transport layer for transport the carriers produced in the carrier generation layer, and a surface protective layer for protecting the carrier generation layer from external stress are generally used.
In addition, in order to prevent the electrons produced from the carrier generation layer by means other than exposure, e.g., due to thermal excitation, from lowering the retention of the positive charges electrified on the surface, a carrier injection regulating layer consisting of a high Se alloy and having a wide band gap is frequently inserted between the carrier transport layer and the surface protective layer. As to other materials, a high-concentration Te--Se alloy is generally used for the carrier generation layer, amorphous Se material for the carrier transport layer and a low-concentration As--Se alloy for the surface protective layer.
The surface protective layer is an important layer that determines the printing durability of a photoreceptor. A low-concentration As--Se alloy is generally used for the surface protective layer because it has a high thermal expansion as compared with As.sub.2 Se.sub.3. Such a material is used in order to avoid cracking due to differences in thermal expansion coefficients of the surface protective layer and the carrier transport layer, which is usually made of an amorphous Se material having a very large thermal expansion coefficient. Unfortunately, these low concentration As--Se alloys have very poor mechanical strength. Accordingly, such a photoreceptor disadvantageously has an insufficient printing durability.
Since it would be possible to enhance the mechanical strength of the surface protective layer by simultaneously lowering the thermal expansion coefficients of the carrier transport layer and the surface protective layer, an Se--Te--As function separation type photoreceptor for a laser beam printer having a high printing durability has recently been developed. In this Se--Te--As photoreceptor, an As.sub.2 Se.sub.3 alloy is used for both the carrier transport layer and the surface protective layer. Since the outermost surface layer consists of an As.sub.2 Se.sub.3 alloy, such a photoreceptor realizes a high printing durability on the same level as a conventional As.sub.2 Se.sub.3 photoreceptor. However, this photoreceptor has poor thermal resistance. That is, since the thermal expansion coefficients of the underlayers, namely, the carrier generation layer and the carrier injection regulating layer are twice that of the surface protective layer, when the photoreceptor is stored at a temperature of 50.degree. C., the carrier generation layer and the carrier injection regulating layer largely expand, thereby producing cracking in the surface protective layer.
Accordingly, it is an object of the present invention to eliminate the above-described defects in the prior art and to provide an electrophotographic photoreceptor having a high printing durability and a high thermal resistance without deterioration of various properties required of a photoreceptor.